The invention relates to an arrangement for fastening an inner conductor of a resonator structure, the arrangement comprising a fastening surface structure comprising an edge of an opening and an opening defined by the edge, the opening being formed through the fastening surface structure, and an inner conductor of a resonator structure at the opening formed through the fastening surface structure, the inner conductor comprising an inner space at least in the portion facing the fastening surface structure, which inner space is defined by the wall of the inner conductor, the wall extending into the opening formed through the fastening surface structure
The invention further relates to a method for fastening an inner conductor of a resonator structure.
Resonator structures employed in high frequency areas, particularly in radio frequency areas, are used for example at base stations of mobile networks. Filters can be used for example as interface circuits and filter circuits in amplifiers of base station transmitter or receiver units.
In the resonator structures the inner conductor of the resonator is fastened to a fastening surface, which is in practice usually the end, such as a bottom or a cover, of a housing structure serving as the outer conductor of the resonator structure. In other words, the inner conductor is short-circuited to the fastening surface, i.e. in practice to the outer conductor. The short-circuited end of the inner conductor, i.e. the end where the inner conductor is short-circuited to the outer conductor, is also called an inductive end, because signal connection takes place inductively. At the other end of the inner conductor, the inner conductor is galvanically isolated from the outer conductor, the other end thus being what is known as a free end of the inner conductor. The free end of the inner conductor is also called its capacitive end, because signal connection takes place capacitively. The outer conductor and the inner conductor inside the compartment it forms together build a resonance circuit. In practice resonator structures are often multi-circuit structures, i.e. the resonator structure comprises a plural number of inner conductor and outer conductor pairs, each compartment formed of an outer conductor having a separate inner conductor. The resonance circuits of a multi-circuit resonator structure together provide the resonator structure with a desired frequency response.
In a prior art solution the inner conductor is fastened to the fastening surface by means of a screw or a bolt. To ensure that the joint between the inner conductor and the fastening surface is tight enough so that problems related to intermodulation are avoided, the solution has to be provided with a weld at the seam between the inner conductor and the fastening surface, i.e. at the foot of the inner conductor. The weld requires that the entire structure is heated in an oven to an elevated temperature, e.g. to 210 degrees Celsius, so it is apparent that the solution is inconvenient and expensive.
Another solution is provided using a riveting tool. The riveting is carried out by hitting the fastening surface around the opening of the fastening surface with the tool at the lower end of the inner conductor to make the material of the fastening surface at the opening squeeze the inner conductor. The problems related to this solution are similar to those described above.
Prior art also knows another riveting-type solution where a spreading tool is used for hitting the inner space of the inner conductor arranged into the opening of the fastening surface, thus causing the inner conductor wall placed in the opening to widen in the lateral direction. This kind of riveting solution does not, however, provide a permanently tight joint, because after the inner conductor wall has been hit with the spreading tool, there is no pressure left to prevent the radial return motion of the wall. The solution is therefore deficient with regard to the prevention of intermodulation and to the strength of the structure, and therefore a weld has to be used.
Fl publication 89115 teaches a structure, shown in FIG. 6 of the publication, where the inner conductor is fastened to an opening formed through the fastening surface with a solid, non-springy insertion piece, the lower edges of the inner conductor being thereby bent against the bottom of the fastening surface. A drawback of the structure is that the insertion piece is solid and non-springy, and therefore the fastening achieved is deficient, particularly with regard to the tightness required to allow intermodulation to be prevented.
An object of the present invention is to provide an arrangement for fastening an inner conductor of a resonator structure in a manner that offers a cost-effective, rapid and convenient means for accomplishing a tight joint with regard to intermodulation and provides a most solid structure by employing, however, a separate inner conductor that is easy to dimension precisely and the surface of which can be made very smooth, the inner conductor being, as stated, fastened to another separate piece, i.e. a fastening surface structure. The dimensional precision of the inner conductor with regard to the smoothness of its surface and its cross-sectional form has, as is well known, a great impact on the characteristics of a resonator structure.
The above stated object is achieved with a fastening arrangement of the invention, wherein the fastening arrangement comprises an expander, springy in the radial direction, which is brought and thereby fastened in place into the inner space of the inner conductor placed into the opening formed through the fastening structure, the expander being used for pressing the inner conductor wall surrounding the inner space of the inner conductor in a radial direction against the edge of the opening formed through the fastening structure at least on the portion of the opening area where the wall surrounding the inner space of the inner conductor coincides with the edge of the opening formed through the fastening surface structure.
A corresponding object is achieved with a method of the invention for fastening an inner conductor, wherein the inner conductor is fastened at an opening in the fastening surface structure by inserting an expander, which is springy in the radial direction, into an inner space of the inner conductor positioned at the opening in the fastening surface structure, the expander being used for pressing the inner conductor wall surrounding the inner space of the inner conductor in a radial direction against the edge of the opening formed through the fastening surface structure at least on the portion of the opening area where the wall surrounding the inner space of the inner conductor coincides with the edge of the opening formed through the fastening surface structure.
The solution of the invention provides a number of advantages. Thanks to the invention, welding or other additional sealing providing intermodulation between the lower end of the inner conductor and the fastening surface, such as the end of the housing structure of the resonator structure, are no longer needed. The end is for example the bottom or the cover of the housing. The solution of the invention alleviates problems related to intermodulation. The preferred embodiments of the invention and other more detailed applications thereof further emphasise the advantages of the invention. In addition, the manufacture of the invention is rapid and can be easily automated.